Common Names

Overview

Biology

Anomalocardia brasiliana is a common bivalve mollusc in shallow, low energy marine environments of the West Atlantic. It ranges from the Caribbean to Uruguay and constitutes an important food item for coastal communities living along the Brazilian coast. The species lives shallowly buried in the sediment, feeds on suspended particles brought into the mantle cavity and trapped by its gills, and has a life span that rarely exceeds 2 years. The shell is triangular, commonly with a produced posterior end. The sexes are separate, fertilization is external and larval development is planktic and rapid.

Common names

Description

Behaviour

Lives shallowly burrowed (approximately 5-15 cm deep) in soft mud, and in muddy beaches in quiet waters. Adult animals bury themselves readily but slowly, with rhythmic movements performed by the muscular foot. When buried, the animal orients itself with the anterior end of the shell pointing downward and the antero-posterior body axis nearly perpendicular to the surface of the sediment. The siphons and sometimes the posterior part of the shell remain exposed at the surface of the sediment. Once buried, the animal remains in the same location for a very long time (Narchi, 1972).

Narchi (1972) observed a specimen measuring 2.5 cm in shell lenght burying itself in 10 minutes, and specimens of 1 cm in 3 minutes. He also reported that animals retain the ability to crawl on the surface of the sediment and observed a specimen measuring 1 cm in shell length crawl 10 cm in 10 minutes.

Genetics

Arruda et al. (2009) recorded signiﬁcant genetic differentiation among four populations of A. brasiliana from the Northeast coast of Brazil, with only 3 out of 36 recognized haplotypes being shared between any pair of populations.

Morphology

Shell: Shell thick, triangular, moderately inflated, up to 45 mm maximum length and variable in shape, coloration and ornamentation. The posterior end is commonly much produced with a pronounced umbonal ridge extending along the posterior slope from the beak to the posterior, ventral margin. Left and right valves equal in size and contour (equivalve). The sculpture consists of very fine, radiating striations and of prominent, commarginal ribs which are developed best anteriorly and over the posterior ridge as well as on the umbonal region. Many large specimens have the area just anterior to the posterior ridge and the ventral region smooth, whereas some shells have the sculpture developed over the whole surface of the shell. Some specimens are shortened posteriorly and the posterior end is more rounded. Internally the shell has a smooth white surface, sometimes with a radial band of purple-brown at the posterior end. The pallial sinus is deep. The muscle scar of the anterior adductor muscle is smaller than that of the posterior adductor muscle. The internal margin of the shell is crenulated (Palmer, 1927–29; Narchi, 1972). Using geometric morphometric methods, Roopnarine et al. (2008) revealed significant differences between Caribbean and Brazilian specimens of A. brasiliana, which are enough to warrant further investigation of the status of these samples as comprising a single species.

Soft parts: The anatomy of the soft parts of A. brasiliana was studied in great detail by Narchi (1972). Below is a summary of his observations:

Siphons: The siphons are short (in an animal measuring 2.47 cm in length, the siphons are 0.4 cm long) and fused up to half their length. The apertures of both siphons are fringed with colorless tentacles, which display white patches on their inner surfaces, giving them a creamlike appearance. In addition to the tentacles, the excurrent siphon has a tapering valvular membrane which, when opened and extended outwards, controls and directs the current of water expelled from the body. The incurrent siphon is relatively wide and its largest nine tentacles are usually bent over the siphon aperture when the animal is pumping water. The excurrent siphon is tubular, with a smaller aperture than the incurrent one.

Mantle: The margin of the mantle has four folds as in all known Veneroidea. The median internal fold is flat, smooth, and has some large pleats. The free edges of the folds of the left and right sides touch each other, closing the foot aperture when the animal is quiescent. When the animal moves, they contact the foot all around.

Gills: Each gill is formed by two demibranchs of which the inner is longer than the outer, especially in the anterior region. Only one food groove is present along the free ventral margin of the inner demibranch. The lamellae of the gills are shallow, plicated, and display morphologically differentiated filaments (heterorhabdic). The plicae are shallow and occur in the greater part of both lamellae with an average number of 16 filaments to a plica, ranging from a minimum of 12 to a maximum of 20 filaments per plicae in the outer and inner demibranchs. Cilliary currents are toward the food groove, where filtered particles are driven to the labial palps and mouth region.

Labial palps: The labial palps are flat and triangular in shape, with the inner faces deeply plicated and the outer faces smooth. The inner demibranchs of the gills project deeply between the palps. Cilliary tracts on the palps drive particles either to the mouth, to be ingested, or to the ventral border of the palps, to be rejected.

Alimentary Tract: The esophagus is short and opens into the anterior ventral wall of the stomach. The combined style sac and intestine opens in the posteroventral region of the stomach and passes backwards and downwards towards the foot. The intestine, from the distal end of this wider tube, describes three closely packed coils on the ventral side of the stomach, and then passes backward to ascend behind the style sac. It then passes through the pericardium, penetrates the ventricle and posterior aortic bulb, and courses dorsal to the posterior adductor muscle, to end in the anal papilla.

Physiology

Able to tolerate large ﬂuctuations in environmental salinity (euryhaline), with little alteration of its hemolymph osmolarity (osmoregulator) (Lima et al., 2009). The species is also able to inhabit areas characterized by low oxygen concentrations and temperatures varying from 15 to 30 degrees Celcius (Boehs et al., 2008).

Ecology and Distribution

Associations

The following organisms were recorded attached to the shell of A. brasiliana (epibionts): algae Enteromorpha sp., barnacles (Cirripedia) and eggs of the snail Nassarius sp. (Prosobranchia) (Narchi, 1972).

A. brasiliana may be found together with other molluscs, such as the bivalves Chione intapurpurea, C. granulata (Veneridae), Eurytellina sp. (Tellinidae), and the snail Nassarius sp. (Nassariidae) (Narchi, 1972).

Dispersal

Dispersal is effected by the planktonic larval stage of the species, which lasts approximately 10 days (Mouëza et al., 1999). The adults are semi-sessile, although they retain the capability of performing short horizontal movements (Narchi, 1972).

Population Biology

The structure of a mangrove population of A. brasiliana was studied by Monti et al. (1991). According to these authors, the species has a high mortality rate early in life, a population size variable in time, a small body size, and a life span rarely exceeding 2 years. The population is multimodal, with more than one generation of clams co-existing, and characterized by a variable recruitment pattern on a year-round basis but with infrequent success. Growth to a stationary average size (of 32 mm) was observed to take 30 months following recruitment. During their 30-month study, Monti et al. (1991) observed four settlements of which only two were successful; the successful settlements coincided with mortality of older individuals, suggesting success is related to diminished competition with a previously established generation. Population densities of up to 1370 indivuduals per square meter were observed at the studied site.

Reproduction

Sexes are separate. Fertilization is external, occurring in the water column (Boehs et al., 2008). Sexual maturation occurs when the species reaches between 12.9 and 17.9 mm in shell length. The species reproduces throughout the year although periods of more intense spawning were observed (Barreira & Araujo, 2005).

Mouëza et al. (1999) reports fecundity of some 100,000 oocytes per female in animals spawned in hatcheries. Oocytes of the species measure 60 microns in diameter, have a thin vitelline coat and no prominent jellycoat. The sperm is very active and characterized by a short arched head (2 microns long and 0.5 micron wide at its base) and a ﬂagellum approximately 40 microns long.

The embryonic and larval development is rapid. The ﬁrst cleavage of the embryo occurs 30 minutes after fertilization, the shell begins to form at the gastrula stage 6 hours after fertilization, and the trochophore and straight-hinged veliger larval stages are reached at 9 and 18 hours after fertilization, respectively. The swimming veligers feed on plankton (planktotrophic development) until day 11, when their apical ﬂagellum and velum begin to regress prior to metamorphosis. Benthic plantigrades with functional gill ﬁlaments develop 15 days after fertilization. In hatchery conditions, it takes seven weeks for the shell of A. brasiliana to acquire its typical triangular shape and pointed posterior end as well as brown zig-zag stripes (Mouëza et al., 1999).

Trophic Strategy

Suspension feeder. Seawater with particles in suspension is pumped through the incurrent siphon and gills, where the particles are retained. They are then sorted by the gills and labial palps. Accepted particles are ingested, whereas rejected matter is agglutinated in mucus and expelled.

Relevance

Uses

Eaten and commercialized, A. brasiliana constitutes an important food item for coastal populations along the Brazilian Coast (Narchi, 1972; Lima et al., 2009). The species is harvested from the wild and not yet cultured (Boehs et al., 2008).